(1) Field of the Invention
The present invention relates to a solid-state imaging device such as a Charge-Coupled Device (CCD) sensor or a Complementary Metal-Oxide Semiconductor (CMOS) sensor and to a method for processing signals outputted from the solid-state imaging device. The present invention particularly relates to a solid-state imaging device provided with a color filter that even enables imaging of a near-infrared light and to a method for processing signals obtained by photoelectric conversion on a light transmitted through the color filter of the solid-state imaging device.
(2) Description of the Related Arts
Solid-state imaging devices conventionally include laminated color filters for transmitting an RGB three-primary-color light (see, for example, Patent Reference 1: U.S. Pat. No. 3,971,065) and enable representation of almost every color.
The conventional color reproduction techniques in which the RGB primary colors are employed, however, have not been sufficient for representing colors as natural as the human eye senses. Particularly, the cyan includes a portion which cannot be represented.
Accordingly, a color filter characteristic has been proposed, which is based on a concept of the XYZ colorimetric system. For the color filter characteristic, it has been taken into consideration that a human optic nerve which senses red has a negative sensitivity to the cyan. Imaging more approaching to the human eye can be achieved by providing a solid-state imaging device with a color filter having such filter characteristic.
It is not possible, however, to obtain the filter characteristic approaching to the characteristic of X of the XYZ calorimetric system simply by superimposing a red-transmission filter and a blue-transmission filter. This is because, when two filters are superimposed, the filter characteristics of the respective filters are included, so that a light is not transmitted through the superimposed filters.
With such a background, a method has been proposed, which implements the negative sensitivity of a red filter by using an emerald filter that transmits only the cyan light which is the subject of the negative sensitivity and by subtracting, from a red signal component, a signal component obtained in a pixel corresponding to the emerald filter (see, for example, Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2003-284084).
Meanwhile, conventional color filters are composed of a pigment or a colorant using organic materials. In the case where these organic materials are used, it is possible to absorb a specific light by utilizing a color separation feature of these organic materials. In this case, the organic materials transmit lights that are not absorbed.
However, a further miniaturized size for a pixel is desired these days as more pixels are included in a light receiving element. This means that the number of electrons to be generated from a received light per pixel of a photodiode has been physically decreased. As described above, when the number of electrons to be generated in a pixel decreases due to the miniaturization, the sensitivity of the photodiode to a light further decreases in the case where the color filter includes the organic materials, since the color filter absorbs a portion of the light.
Accordingly, a reflective color filter has been proposed as a color filter that enables color separation without decreasing light transmission (see, for example, Patent Reference 3: Japanese Unexamined Patent Application Publication NO 2005-101109). More specifically, this reflective color filter uses an inorganic dielectric multilayer filter in which a film with a high refractive index and a film with a low refractive index are alternately laminated and which transmits a light having a particular wavelength under a certain film-thickness condition. Use of such multilayer allows a light forbidden band to be produced, in which a light with a certain wavelength range cannot be transmitted. By superimposing films which are different in structure enables transmitting only a light with a specific wavelength. Use of this phenomenon makes possible the implementation of three-type color filters each of which transmits desired three primary colors.
A luminance signal Y is generally expressed by the following expression: Y=0.11B+0.59G+0.30R. Here, B, C and R represent a blue signal, a green signal and a red signal, respectively. With regard to this, it is known that, in order to obtain a luminance signal with a large value, use of a pixel arrangement called “Bayer arrangement” in which a unit cell includes four pixels of R, G, G, and B enables the most efficient obtainment of the luminance signal (see, for example, Patent Reference 1: U.S. Pat. No. 3,971,065). Further, from the above description, use of an emerald pixel in place of one of the G pixels of the RGGB pixel can be considered.
However, when the emerald pixel is used in place of one of the G pixels of the conventional RGGB pixel, the green signals that dominate in the luminance signal are reduced by half, in other words, from two green signals to one green signal. Accordingly, the luminance signal cannot be obtained efficiently. As stated above, among the human visual sensitivity, particularly the visual sensitivity to red has the negative sensitivity to the light of 500 nm wavelength. There has not been a color filter that has such a filter characteristic as the human visual sensitivity has.